Whether flu or coronavirus, it can take several days for the body to ramp up an effective response to a viral infection.
https://www.eurekalert.org/pub_releases/2020-09/uorm-cst093020.php
origin - https://www.pipiscrew.com/2020/10/cells-sacrifice-themselves-to-boost-immune-response-to-viruses/ cells-sacrifice-themselves-to-boost-immune-response-to-viruses